Wow, you have one of those late D24TIC EGR motors with the extra timing belt tensioner... Cool. Older ones used the water pump (mounted with slotted holes) to set the belt tension... the later ones like this should be easier.
Volvo special tool 9995187 is what is supposed to be used for crank pulley torquing. It grabs the lugs on the inside of the balancer and counterholds it while you reef on it with a cheater bar to get about 350# of torque on the bolt. Not too many other alternatives, if you want to actually torque it to the spec, you pretty much have to have the tool, or make a homemade copy (as some on here have done). Here in the US the tool is pretty much unobtainium unless you luck out at a Volvo indy shop or dealer that has it and is willing to let it go or can borrow it from a forum member here. May be easier to track one down in the UK since anyplace that does serious work on VW LT vans with this engine should also have one.
You might contact 745 TurboGreasel for some dimensional info on the homebrew tools he made; if you are handy with a welder or know someone who is, that may be your easiest option.
Original tool looks like this, also pictured is wrench extension 9995188 which makes the job easier but can be done without:
